Aluminum casting alloy

ABSTRACT

An aluminum alloy that is controlled in composition with respect to aluminum, major alloying element, grain refiner, trace element, and impurity contents to develop increased tensile strength properties in an as-cast, heat-treated, and aged condition, particularly when directionally solidified into casting configurations having a Shape Factor substantially in excess of 100.

United States Patent Inventor Herbert Greenewald, Jr. [56] References Cited Columbus, Ohio UNITED STATES PATENTS 9f 1 1 22 3,198,676 8/1965 Sprowlsetal 1971 2,240,940 5/1941 Nock i 2,301,759 11/1942 Stroup Assignee North American Rockwell Corporation 2865 796 12/1958 Rosenkranz Continuation-impart ofapplieation Ser.No. 3287'l85 H1966 vachet at 614,838, Feb. 9, 1967, now abandoned. Primary ExaminerRichard 0. Dean Attorneys-William R. Lane and Daniel H. Dunbar ALUMINUM p ALLOY ABSTRACT: An aluminum alloy that is controlled in com- 7 clamssnnw gngs' position with respect to aluminum, major alloying element, US. Cl 148/32, grain refiner, trace element, and impurity contents to develop 75/141, 75/146, 148/325, 148/159 increased tensile strength properties in an as-cast, heat- Int. Cl C22c 21/00 treated, and aged condition, particularly when directionally Field of Search 75/139, solidified into casting configurations having a Shape Factor PERCENT COPPER, BY WEIGHT substantially in excess of 100.

M!" limit [7. Cu 43.6% I. Mg] (Approximately) I. Cu [3.9 I. I. M9] (Approximately) l l l 2 3 4 5 6 7 8 9 PERCENT MAGNESIUM, BY WEIGHT "I-Cu I 0.6% (Approximately) M9 1.0% (Approxlmulely) PAIENIEDIIUV l6 l97l 3. 620.854

SHEET 5 BF 5 3 AHZ AI+Z+S +2 2 A) A I I I Al+ -CuAI Al+S PERCENT COPPER, BY WEIGHT AI+$+T Melt limit Cu =[3.6/ Mg] (Approximately) Cost olloy limit Cu [3.9% Mg] (Approximately) AI-I-T 3 4 5 6 7 8 9 PERCENT MAGNESIUM, BY WEIGHT %Cu 0.6% (Approximately) Mg LO (Approxlmolely) IN VENTOR.

HERBERT GREEN EWALD, JR.

ATTORNEY ALUMINUM CASTING ALLOY CROSS-REFERENCE This application is a continuation-in-part of pending application Ser. No. 614,838, filed Feb. 9, I967 (now abandoned) and assigned to the assignee of this application.

SUMMARY OF THE INVENTION The alloy of this invention is controlled in composition to have, on a weight basis: (1) aluminum substantially in the range of 84.2 percent to 93.2 percent; (2) major alloying elements from the group consisting of zinc, magnesium, and copper substantially in the range of 5.6 percent to 12.0 percent; (3) grain refiners consisting of titanium and beryllium substantially in the range of 0.2 percent to 0.6 percent; (4) trace elements, including chromium and manganese, to 0.8 percent maximum; and (5) impurities, including silicon and iron, not exceeding substantially 0.25 percent maximum. Particular alloys in the composition range, when directionally solidified into casting configurations having a Shape Factor significantly greater than 100 and when in an as-cast (nonworked), heat-treated, and aged condition, have developed ultimate tensile strengths as great as 92,200 p.s.i., yield tensile strengths (0.2 percent offset) of up to 78,400 p.s.i., or elongations (in 2 inches) as great as 9.6 percent.

DESCRIPTION OF THE DRAWINGS FIGS. 1(a) and 1 (b) comprise a sequential flow diagram for a controlled melting, casting, directional solidification, and heat-treating process that may be utilized to advantage in connection with the aluminum casting alloy of this invention.

FIGS. 2(a) and 2b) comprise a sequential flow diagram for a controlled directional solidification and heat-treating process that may be utilized to advantage in connection with those species of the aluminum alloy of this invention having a reduced maximum trace element content and having a copper-magnesium content restriction that permit melting and casting in a noncontrolled atmosphere and also directional solidification with temperature arrests for a substantial time during cooling above the alloy solidus temperature.

FIG. 3 is an aluminumcopper-magnesium-6 percent zinc alloy phase diagram showing the limits of a copper and magnesium content relationship preferred for species of the aluminum casting alloy of this invention subjected to directional solidification with a substantial temperature arrest above the alloy solidus temperature.

DETAILED DESCRIPTION Constituents Percent Weight Major alloying elements from group Consisting of zinc,

magnesium and copper 5.6% to l2.0% Grain refiners consisting of titanium and beryllium Trace elements (including chromium and manganese) Impurities (including silicon 0.8% maximum and iron) 0.25% maximum Aluminum Balance TOTAL l00.00%

Within the foregoing ranges, it is preferred that: (l) the major alloying elements consist of zinc in the range of substantially 4.0 percent to 8.0 percent magnesium in the range of substantially 1.0 percent to 3.5 percent, and copper in the range of substantially 0.6 percent to 2.9 percent; (2) the grain refiners consist of titanium in the range of substantially 0.l percent to 0.3 percent and beryllium in a like range; (3) the trace elements include not more than substantially 0.4 percent chromium and not more than substantially 0.4 percent manganese; and (4) the impurities include not more than 0.2 percent iron (maximum) or 0.05 percent silicon (maximum). The balance of the composition consists of aluminum.

Examples of four different alloys having a composition in accordance with this invention are given in the following table I; the included numerical values are constituent amounts on a percent of total weight basis:

The foregoing alloy compositions, if controlled in their melting, casting, directional solidification and heat-treating prior to aging, will develop tensile strength properties in the as-cast, heat-treated, and aged condition which compare favorably with the corresponding properties developed in representative wrought, heat-treated, and aged condition conventional high-strength aluminum alloys. Further, the tensile strength properties of the foregoing aluminum casting alloy composition are distinctly superior to the corresponding properties developed in representative as-cast, heat-treated, and aged condition conventional aluminum casting alloys. More specifically, the above-defined alloy compositions have been cast into thinwalled configurations to achieve, in combination, a minimum ultimate tensile strength of approximately 76,000 p.s.i., a minimum yield tensile strength (0.2 percent offset) of approximately 70,000 p.s.i., and a minimum elongation (in 2 inches) of approximately 2 percent. Such properties compare favorably with the minimum ultimate tensile strength of 78,000 p.s.i., minimum yield tensile strength of 70,000 p.s.i., and minimum elongation of 7 percent developed in wrought (mechanically worked), heat-treated, and aged condition conventional high-strength aluminum alloys such as alloy 7075 in comparable sections. Conventional aluminum casting alloys such as numbers 354, C355, A356, A357, and 359 develop corresponding properties of 47,000 p.s.i. minimum ultimate tensile strength, 36,000 p.s.i. minimum yield tensile strength, and 5 percent minimum elongation for the as-cast but heat-treated and aged condition. A flow diagram of one suitable method for controlling melting, casting, directional solidification and heat-treating of the novel aluminum casting alloy prior to aging is provided in FIG. I.

As suggested by FIG. I, the preferred controlled procedure for processing the aluminum casting alloy compositions of this invention to obtain increased tensile strength properties in the as-cast, heat-treated, and aged condition is essentially comprised of the steps numerically designated 11 through 23. The aging and post-aging cooling steps referenced as 22 and 23 in the process are entirely conventional. However, important differences over conventional practices have been developed with respect to steps 11 through 21. It should be noted that alternate sequences are available with respect to accomplishing the steps sequentially intermediate blocks I3 (First-stage Cooling) and 20 (Solution and l-lomogenization Heat-treating). Different of the alternate sequences may be preferred depending on hereinfter-described limitations; in any event, the desired end-results are obtained regardless of which one of y the disclosed alternate sequences is selected.

a that further is hydrogen-free as well as moisture/hydrocarbonfree. Pure argon having less than 100 parts per million (p.p.m.) of water and/or hydrocarbons, for example, meets the requirements of a controlled atmosphere. With respect to pressure, the steps of melting ll, pouring l2, first-stage cooling l3, and solution and homogenization heat-treating 20 must be accomplished in connection with an environmental pressure of inches of mercury (absolute) or above. Thus, typical vacuum casting conditions are intolerable in connection with steps ll, l2, l3, and 20, and are even preferably avoided in connection with all of the other steps of the FIG. 1 process.

The melting step referenced as 11 must be accomplished in .an environment having a controlled atmosphere and having a minimum pressure of 15 inches of mercury (absolute). The aluminum casting alloy of this invention is melted to a temperature in the range of 1250 F. to 1350 F. prior to pouring. It is also necessary that the crucible apparatus utilized for melting be nonreactive with respect to the alloy and also be free of included or occluded alloy impurities, water, and hydrocarbons. Degassed, high-purity ATJ graphite is a satisfactory crucible material.

Pouring step 12 must also be accomplished within an environment having a controlled atmosphere and a controlled pressure (15 inches of mercury (absolute), minimum). In accomplishing pouring, the alloy temperature should not be less than the 1250 F. minimum temperature specified in connection with melting (step 11). The casting mold into which the molten alloy is poured must, as in the case of the crucible apparatus referenced above, be nonreactive with respect to the alloy and must be free of impurities, moisture, and hydrocarbons, whether included or occluded. The above-suggested high-purity graphite, provided with a machined internal cavity and riser configuration, is a suitable mold material. Also, refractory oxide particles may be molded to the desired configuration and afterwards carbonaceous-bonded to form a nonreactive casting mold. As indicated by copending application Ser. No. 498,046, filed Oct. l9,'l965, (now abandoned() -325 mesh zircon flour mixed with a conventional thermosetting phenolic resin may be molded with the required internal cavity and riser configuration and afterwards properly fired to develop a satisfactory casting mold. Also, it is important that the casting mold be heated to a temperature above the alloy liquidus curve (in the case of table I alloys, above approximately l,200 F.) at the time of pouring; such is normally accomplished through the use of cooperating electrical resistance heating elements secured to the casting mold in heattransferring relation.

First-stage cooling, the step referenced by block 13, is accomplished during the interval between completely filling the mold casting and riser cavity with molten metal at l,250 F. to completing casting solidification and cooling to a temperature in the range of 800 F. maximum to 600 F. minimum. As in the case of melting step 11 and pouring step 12, the first-stage cooling accomplished in step 13 must occur within an environment having a controlled atmosphere and having pressure in excess of 15 inches of mercury (absolute). The alloy initiai temperature is the range of the L350 F. maximum melting temperature to the l,200 F. minimum mold temperature specified in connection with steps 11 and 12; the final firststage cooling temperature is in the range of 600 F. to 800 F. with final temperatures in the lower portion of the range being preferred in most instances. It is important to note that the rate of cooling is carefully controlled, as by the selective operation of cooling apparatus and heating apparatus cooperatively combined with the casting mold, in the interval between the specified initial and stopping temperatures. It is important that cooling in the first-stage cooling step be accomplished so that the alloy solidification front moves progressively either in a continuous or incremental manner, from the casting portion furthermost from the riser to the riser. Such must be accomplished with care that the temperature of the casting (or casting mold) at no time is below 600 F. In general, the required solidification is achieved if the minimum temperature of the alloy solidification front range (l,l F. to 810 F.) occurs in the casting in the prescribed progression. Accelerated heat removal from mold portions most distant from the riser, in combination with supplemental heating in mold regions more adjacent to the casting riser, is generally required to accomplish the specified controlled cooling. Copending applications Ser. Nos. 507,384 and 508,318, assigned to the assignee of this application, disclose apparatus that may be operated to attain the specified cooling. Thin-walled (e.g., 0.060 inches thick) casting sections formed of the alloy of this invention and having a 15 inch path from locations farthermost from the casting riser to locations in the riser may require as much as 6 hours of controlled cooling to obtain the desired solidification directionality.

The process steps utilized immediately subsequent to step 13 but prior to solution and homogenization heat-treating (step 20) are selected from the alternate sequences of blocks 14 through [9 largely on the basis of the characteristics of the mold utilized and the casting removal techniques that are available to advantage. If it is impractical to remove the solidified casting from the mold at elevated temperatures (e.g., at temperatures above 600 F. 700 F. minimum) steps 14 through 16 may be selected to follow the indicated firststage cooling but with an incurred time penalty. Sequence of steps 14 through 16 requires, as a minimum, considerably more time for accomplishment than do the alternate sequences indicated by blocks 17 through l9. If the solidified alloy can advantageously (conveniently) be removed from the casting mold at elevated temperatures, either of two process variations may be involved. If the mold is to be reused, as is often the case in connection with graphite molds, the casting is ejected from the mold in connection with step 17 by conventional mechanical ejector means. Since graphite molds and the like are oxidizable at temperatures above approximately 400 F., step 17 is preferably accomplished in a controlled atmosphere; the removal step may be accomplished at ambient atmospheric pressures, however, if desired. In any event, it is necessary that the casting be removed from the mold before the solidified alloy has cooled to below the specified 600 F. minimum. Depending upon casting configuration characteristics, step 17 may be accomplished at alloy temperatures to as high as approximately 800 F.

The alternate casting removal step designated 18 must also be accomplished at a temperature in excess of 600 F. It is selected in instances wherein the mold is fabricated of an oxidizable material and the casting removal is to be accomplished by mold oxidation and disintegration. ln such instances step 18 is accomplished in an ambient atmosphere and at ambient pressures merely by transfer of the 600 F. 800 F. casting and mold combination from the controlled atmosphere to air or an oxygen equivalent medium above the minimum removal temperature.

If either step 17 or 18 is selected for alloy processing immediately following first-stage cooling, the subsequent second-stage cooling is accomplished as shown by block 19. Such postsolidification cooling may be conveniently accomplished in ambient atmospheres at ambient pressures. The cooling starting temperature for the removed alloy casting is in the previously stated range of 600 F. to 800 F. and the final temperature is ambient (room) temperature. Cooling rates are normally those obtained by cooling the removed casting by immersion in the ambient temperature atmosphere.

lf casting removal cannot be accomplished at elevated temperatures as in connection with step 17 or 18, annealing step 14 is selected to follow first-stage cooling 13). In the annealing operation tlte casting and mold combination is maintained at a temperature in the range of 500 F. to 600 F. for sufficient time to develop increased ductility in the casting. For the alloy compositions identified in table I, heating for 5 hours at 500 F. subsequent to first-stage cooling is nonnally adequate. Ambient atmospheres and ambient pressures may be utilized.

After annealing step 14 is completed, second-stage cooling in the alternate sequence is accomplished as indicated by block 15 and the casting subsequently removed from the mold as indicated by block 16. Second-stage cooling step 15 corresponds to second-stage cooling step 19 with respect to absence of critical limitations except that the starting temperature is in the range of 500 F. to 600 F. rather than 600 F. to 800 F. The removal of the completed casting from the mold is accomplished in connection with step 16 by forceful ejection in a conventional manner or by disintegration of the mold.

In order to develop the optimum strength characteristics for the aluminum casting alloys of this invention the cast alloy should be heat-treated and aged after removal from the mold. Heat-treating is accomplished essentially in accordance with the critical limitations specified for step 20. it is important that such heat-treating be accomplished in an environment with a controlled atmosphere and with a pressure above the prescribed minimum pressure of 15 inches of mercury (absolute). Time and temperature parameters, however, are essentially conventional and for the alloys specified in table I, 72 hours at 860 F. i 40 F. is generally preferred. Time periods to as little as 20 hours have sometimes been used. Generally, the longer time periods are preferred. The step is accomplished so as to eliminate second-phase precipitates at alloy grain boundaries.

The post-heat-treat cooling step 21 preferably occurs at ambient temperatures. Since the cooling rate must be sufficiently high to obtain a nonequilibrium metallurgical structure, the cooling rate must involve a comparatively high rate of heat transfer. As indicated in FIG. 1, it is preferred that the alloys of this invention be quenched from the solution and homogenization heat-treating temperature to ambient temperature in ambient temperature water. This differs from the nonnal practice of accomplishing post-heat-treat cooling (step 21) in heated water such as l80 F for instance.

The steps identified by blocks 22 and 23 are conventional accelerated aging steps. In terms of the aluminum alloy casting of this invention, accelerated aging temperature-time histories in the range of from 450 F. for 2 hours to 250 F. for 24 hours are adequate. No environmental controlled atmosphere or environmental controlled pressure is required. Cooling in accordance with step 23 is conventional and may be accomplished by immersing the acceleration aged alloy in the ambient temperature environmental atmosphere.

Aluminum alloy castings have thin-walled sections (e.g., 0.060 inch thickness) have been made with the aluminum casting alloys of this invention and in accordance with the controlled procedures described herein. Tensile strength and elongation property tests were performed in a conventional manner with respect to such specimens. The obtained data is set forth in the following table II:

All of the specimens were tested at room temperature in the as-cast condition. Also, all of the specimens were processed in accordance with the sequence of steps 11 through 12 and 20 through 23. Different specific times and temperatures, however, were utilized in connection with the solution and homogenization heat-treating of step 20 and the aging of step 22. More specifically, cast sections of the Melt 7-5 material were solution heat-treated 19.5 hours at 880 F. and then 24.0 hours at 920 F.; the Melt 7-5 specimens were aged for 24 hours at 300 F. Intermediate cooling was by water quenching to room temperature from 920 F. With respect to Melt J and Melt K specimens, the time and temperature for solution and homogenization heat-treating was 72 hours at 920 F. Each of the Melt .l and Melt K specimens were acceleration aged at 250 F. for 24 hours, and intermediate cooling was by water quenching to room (ambient) temperature. Melt F specimens tested and reported above were solution and homogenization heat-treated in air for 8 hours at 880 F. followed by 12 hours at 920 F.; the low elongation value and below-minimum yield tensile strength are attributable, at least in part, to the failure to adhere to the stated processing critical limitations as to atmosphere conditions.

It should be emphasized that the master melt examples of the casting alloy of this invention previously detailed are particularly well suited to directional solidification into thinwalled configurations (i.e., configurations with a Shape Factor significantly greater than using process steps of melting and pouring in a composition-controlled atmosphere and also of directionally solidifying the alloy without prolonged anestment above the alloy solidus temperature. However, particular species of the aluminum alloy of this invention may also be utilized advantageously in connection with process steps of melting and casting in an air (noncontrolled) atmosphere and of directionally solidifying with prolonged arrestment above the alloy solidus temperature such as to accomplish a degassing objective through vacuum/pressure cycling.

Such alternate processing is summarized in the flow diagram of FIGS. 2(a) and 2(b); the species of the aluminum casting alloy of this invention best suited to such processing are characterized by a restriction relating to the magnesium and copper constituents. More particularly, copper should be present in the directionally solidified alloy in the range of 0.6 percent to 2.9 percent approximately but not exceeding the quantity 3.9 percent minus the percent magnesium content, and magnesium in the directionally solidified alloy is restricted to the range of 1.0 percent to 3.3 percent approximately but not exceeding the quantity 3.9 percent minus the percent copper content. Because of metallographically observed macrosegregation associated with arrested directional solidification above the alloy solidus temperature, it is believed generally desirable to further restrict the copper content and magnesium content relationship in the master melt to copper in the range of 0.6 percent to 2.6 percent but not exceeding 3.6 percent minus the percent magnesium content and to magnesium in the range of 1.0 percent to 3.0 percent but not exceeding 3.6 percent minus the percent copper content. The phase diagram of FIG. 3 illustrates the preferred restriction by shading and by approximate straight line boundaries. Also, and equally important, the alloy species particularly well suited to FIG. 2 processing, are characterized by chromium and manganese contents of less than 0.05 percent.

Examples of three different directionally solidified alloys substantially in accordance with the species restriction are given in the following table "I; the included numerical values are constituent amounts on a percent of total weight basis:

Titanium 0.20 0.20 0.15 Beryllium 0.16 (H6 0.

Chromium 0.01 0.01 0.01 Manganese 0.0I 0.0] 0.01

Iron 0.09 0.04 0.09 Silicon 0.10 010 0.09

Aluminum Balance Balance Balance TOTAL 100.00% 100.00% 100.00%

The previously detailed Master Melt .I composition is also considered to be within the composition range of the species.

The foregoing table "I alloy compositions, if controlled in their metal solidification and heat-treating prior to aging, may be processed to develop tensile strength properties in the ascast (nonworked), heat-treated, and aged condition that also compare favorably with the corresponding properties developed in representative wrought, heat-treated, and aged condition conventional high-strength aluminum alloys. More specifically, the above detailed alloy species have been cast into thin-walled configurations to achieve, in combination, an ultimate tensile strength of approximately 78,500 psi. a yield tensile strength (0.2 percent offset) of approximately 66,500 p.s.i., and an elongation (in 2 inch) of approximately 9.6 percent. A flow diagram for the suitable method for controlling metal solidification and heat-treating of the aluminum casting alloy prior to aging (and without melting or casting in a controlled atmosphere) is provided in FIG. 2.

As suggested by FIG. 2, the preferred controlled procedure for processing the aluminum casting alloy compositions of this invention to obtain increased tensile strength properties in the as-cast, heat-treated, and aged condition is essentially comprised of the steps numerically designated 111 through 123. The melting and pouring steps referenced as 111 and 1 l2 and the aging and postaging cooling steps referenced at 122 and 123 in the process are entirely conventional. However, important differences over conventional practices have been developed with respect to steps 113 through 121. It should be noted that alternate sequences are available with respect to accomplishing the steps sequentially intermediate blocks 113 (First-Stage Cooling) and 120 (Solution and Homogenization Heat-treating). Different of the alternate sequences may be preferred depending on hereinafter described limitations; in any event, the desired end-results are obtained regardless of which one of the disclosed alternate sequences is selected.

Throughout the following detailed description of the FIG. 2 method, frequent reference is made to a controlled atmosphere and to a controlled pressure. As used herein, controlled atmosphere as in the case of FIG. 1 means an environment that compositionwise is nonreactive with respect to the aluminum casting alloy and that further is hydrogen-free as well as moisture/hydrocarbon-free. Pure argon having less than I parts per million (p.p.m.) of water and/or hydrocarbons, for example, meets the requirements of a controlled atmosphere. With respect to controlled pressure, the steps of first-stage cooling I13 and solution and homogenization heattreating 120 involve vacuum atmospheres to as low as 2 X mm. Hg and also (in the case of step 113) repressurization to above standard atmospheric pressure, such as to 15 p.s.'i.g. by way of example, in the controlled content atmosphere.

The melting step referenced as lll may be accomplished in a normal ambient air environment. The aluminum casting alloy of this invention is melted to a temperature in the range of l,250 F. to 1,350" F. prior to pouring. It is also highly desirable that the crucible apparatus utilized for melting be nonreactive with respect to the alloy and also be free of included or occluded alloy impurities, water, and hydrocarbons. Degassed, high-purity ATJ graphite is a satisfactory crucible material.

Pouring step 112 may also be accomplished within a normal ambient oxygen-containing air environment. In accomplishing pouring, the alloy temperature should not be less than the 1,250 F. minimum temperature specified in connection with melting (step Ill). The casting mold into which the molten alloy is poured must, as in the case of the crucible apparatus referenced above, be nonreactive with respect to the alloy and must be free of impurities, moisture, and hydrocarbons, whether included or occluded. The above-suggested high-purity graphite, provided with a machined internal cavity and riser configuration, is a suitable mold material. Also, refracto ry oxide particles may be molded to the desired configuration and afterwards carbonaceous-bonded to form a nonreactive casting mold. As indicated by copending application Ser. No. 498,046, filed Oct. 19, I965 (now abandoned), -325 mesh zircon flour mixed with a conventional thermosetting phenolic resin may be molded with the required internal cavity and riser configuration and afterwards properly fired to develop a satisfactory casting mold. Also, it is important that the casting mold be heated to a temperature above the alloy liquidus curve (in the case of table I alloys, above approximately I,200 F.) at the time of pouring; such is normally accomplished through the use of cooperating electrical resistance heating elements secured to the casting mold in heat-transferring relation.

First-stage cooling, the step referenced by block 113, is accomplished during the interval between completely filling the mold casting and riser cavity with molten metal at l,250 F. to completing casting solidification and cooling to a temperature in the range of 800 F. maximum to 600 F. minimum. The first-stage cooling accomplished in step 113 preferably occurs within an environment that is cycled at prescribed alloy temperature conditions to a reduced pressure of 5 X 10" mm. Hg and then to a controlled atmosphere condition with a pressure in excess of standard atmospheric pressure (e.g., to 15 p.s.i.g.). The alloy initial temperature is the range of the approximately l,350 F. maximum melting temperature to the l,200 F. minimum mold temperature specified in connection with steps Ill and 112; the final first-stage cooling temperature is in the range of 600 F. to 800 F. with final temperatures in the lower portion of the range being preferred in most instances. Environment cycling is accomplished at least twice before any of the molten metal is cooled to below 1,200 F., and also is accomplished at least once (and preferably twice) while the metal is in the temperature range of l,025 F. to l,l50 F. It is important to note that the rate of cooling, even with the temperature arrest for vacuum/pressure cycling, may be carefully controlled by the selective operation of cooling apparatus and heating apparatus cooperatively combined with the casting mold in the interval between the specified initial and stopping temperatures. It is important that cooling in the first-stage cooling step be accomplished so that the alloy solidification front moves progressively from the casting portion furthermost from the riser to the riser. Such must be accomplished with care that the temperature of the casting (or casting mold) at no time is below approximately 600 F. There normally is an arrest in the total temperature progression at a temperature in the range of 1,02520 F. to 1,] 50 F. to permit the required concurrent vacuum/pressure cycling stated at block 113 of FIG. 2. In general, the required solidification is achieved if the minimum temperature of the alloy solidification front range (I,l75 F. to 8I0 F.) occurs in the casting in the prescribed progression. Accelerated heat removal from mold portions most distant from the riser, in combination with supplemental heating in mold regions more adjacent to the casting riser, is generally preferred to accomplish the specified 7 controlled cooling. Copending applications Ser. Nos. 507,384

(granted July 23, 1968 as U.S. Letters Pat. No. 3,393,836) and 508,318 (now abandoned), assigned to the assignee of this application, disclose apparatus that may be operated to attain the specified cooling.

The process steps utilized immediately subsequent to step 113 but prior to solution and homogenization heat-treating (step 120) are selected from the alternate sequences of blocks 114 through 119 largely on the basis of the characteristics of the mold utilized and the casting removal techniques that are available to advantage. The previous comments developed with respect to steps 14 through of FIGS. 1(a) and 1(b) also pertain to the processing of blocks 114 through 120 of FIG. 2 and therefore are not repeated here.

In order to develop the optimum strength characteristics for the aluminum casting alloys of this invention the cast alloy should be heat-treated and aged after removal from the mold as indicated by block 120. It is important that such heat-treating be accomplished in a vacuum environment, such as an atmosphere with a pressure of 2 X 10" mm. Hg. For the alloys specified in table 111, 24 hours at 860 F. 1 40 F. per 0.1 inch of metal thickness is generally satisfactory with respect to time and temperature. The step is accomplished so as to eliminate entrapped hydrogen and second-phase precipitates at alloy grain boundaries.

The post-heat-treat cooling step 121 preferably occurs at ambient temperatures. Since the cooling rate must be sufficiently high to obtain a nonequilibrium metallurgical structure, the cooling rate must involve a comparatively high rate of heat transfer. As indicated in FIG. 2, it is also preferred that the alloys of this invention be quenched from the solution and homogenization heat-treating temperature to ambient temperature in ambient temperature water. This differs from the normal practice of accomplishing post-heat-treat cooling (step 121) in heated water such as 180 F., for instance.

The steps identified by blocks 122 and 123 are conventional accelerated aging steps. In terms of the aluminum alloy casting of this invention, accelerated aging temperature-time histories in the range of from 450 F. for 2 hours to 250 F. for 24 hours are adequate. No environmental controlled atmosphere or environmental controlled pressure is required. Cooling in accordance with step 123 is conventional and may be accomplished by immersing the acceleration aged alloy in the ambient temperature environmental atmosphere.

Aluminum alloy castings having thin-walled sections (e.g., 0.060 inch thickness) as well as with thicker (1 inch) sections have been made with the aluminum casting alloys of table 111 and in accordance with the controlled procedures of FIG. 2. Tensile strength and elongation property tests were performed in a conventional manner with respect to such specimens. The obtained data is set forth in the following Table IV:

offset), .s.i. 77,900 66,800 Elongation (in 2"),

percent I .6 3.0 2.6

All of the specimens were tested at room temperature in a nonworked condition. Also, all of the specimens were processed in accordance with the sequence of steps 111 through 112 and through 123, except that step 120 for coupon 4-7 involved an argon atmosphere at standard pressure. This difference is considered to account for the reduced ductility (elongation).

1 claim:

1. A directionally solidified aluminum alloy consisting, on a percentage weight basis, of:

a. Aluminum in the range of 86.9 percent to 92.8 percent;

b. Zinc, magnesium, and copper, in the total range of approximately from 7.0 percent to 12.0 percent; c. Titanium in the range of approximately from 0.1 percent to 0.3 percentd. Beryllium in the range of approximately from 0.1 percent to 0.3 percent;

e. Chromium and manganese in the total range of approximately from 0 to 0.8 percent; and

f. Iron and silicon in the total range of approximately from 0 to 0.25 percent.

2. The aluminum alloy defined by claim 1, wherein said zinc is in the range of 4.5 percent to 3.5 percent, and said copper is in the range of 1.0 percent to 2.0 percent, and wherein said chromium and said manganese are less than approximately 0.4 percent.

3. The aluminum alloy defined by claim 2, wherein said iron and said silicon total less than approximately 0.1 percent.

4. The aluminum alloy defined by claim 2, wherein said chromium and said manganese total less than approximately 0.02 percent.

5. The aluminum alloy defined by claim 4, wherein said iron and said silicon total less than approximately 0.1 percent.

6. The aluminum alloy defined by claim 1, wherein said zinc is in the range of 4.0 percent to 8.0 percent approximately, said magnesium is in the range of 1.0 percent to 3.3 percent approximately, and said copper is in the range of 0.6 percent to 2.9 percent approximately, said percentage weight of copper being less than approximately 3.9 percent minus the percentage weight of said magnesium and said percentage weight of magnesium being less than approximately 3.9 percent minus the percentage weight of said copper.

7. The aluminum alloy defined by claim 6, wherein said percentage weight of copper is less than approximately 3.6 percent minus the percentage weight of said magnesium and said percentage weight of magnesium is less than approximately 3.6 percent minus the percentage weight of said copper. 

2. The aluminum alloy defined by claim 1, wherein said zinc is in the range of 4.5 percent to 3.5 percent, and said copper is in the range of 1.0 percent to 2.0 percent, and wherein said chromium and said manganese are less than approximately 0.4 percent.
 3. The aluminum alloy defined by claim 2, wherein said iron and said silicon total less than approximately 0.1 percent.
 4. The aluminum alloy defined by claim 2, wherein said chromium and said manganese total less than approximately 0.02 percent.
 5. The aluminum alloy defined by claim 4, wherein said iron and said silicon total less than approximately 0.1 percent.
 6. The aluminum alloy defined by claim 1, wherein said zinc is in the range of 4.0 percent to 8.0 percent approximately, said magnesium is in the range of 1.0 percent to 3.3 percent approximately, and said copper is in the range of 0.6 percent to 2.9 percent approximately, said percentage weight of copper being less than approximately 3.9 percent minus the percentage weight of said magnesium and said percentage weight of magnesium being less than approximately 3.9 percent minus the percentage weight of said copper.
 7. The aluminum alloy defined by claim 6, wherein said percentage weight of copper is less than approximately 3.6 percent minus the percentage weight of said magnesium and said percentage weight of magnesium is less than approximately 3.6 percent minus the percentage weight of said copper. 